Heating, ventilating and air conditioning systems sometimes use a variable frequency drive (VFD) to control the operation of a motor, such as a variable speed blower motor. The VFD modulates the frequency and voltage provided to the motor to modulate the power and speed of the motor. The VFD and the motor thus produce a controllable airflow that may be used to provide more precise conditioning of a conditioned space and/or power savings relative to similar systems employing a single-speed blower motor.
VFDs, like most electronic device, may fail after a period of normal operation. In an HVAC system such failure may result in loss of an HVAC service, e.g. heating or cooling. Thus, in some HVAC applications a bypass circuit may bypass the VFD to provide power to the blower motor to maintain the HVAC service until the VFD can be repaired or replaced.